I just found myself in a situation where I wanted to breakout all the connection on a Keithley 238 to banana jacks for everyday use, where extreme sensitivity and leakage is not a big concern.
Sadly, it was actually cheaper for me to buy 3x triax cables, a Hammond box, panel mount triax connectors, and some banana jacks than it was to buy a pre-made solution. You may find like I did, that it is just cheaper to build something yourself.
I looked at those adapters that remove the guard connection and they are ridiculously expensive. I would say that the ones that tie the guard to the outer shield could have their uses though. You could use something like a Pomona 5229 (~$10) to Pomona 1894 (~$10) and have a fairly simple way to bring the center conductor to a single banana connection that maintains the guard through the entire length of the cable (up to the Pomona 1894).
I have a simple shielded test fixture I built where I can just jumper the connections together between different devices. The test fixture has triax, banana jacks, and I plan to add a couple bnc connectors in the future. Not as simple (and small) as a nice adapter, but it does what I need.
A small metal box that has a triax connector on one end, a bnc on the other, where you run a wire between the two center conductors may be cheaper than an adapter.
There are a lot of people here with more experience than me though, so maybe they will have a better solution for you.